Let me tell you that I would LOVE to be able to add this feature to the classifieds.
I will ask around and see if I can find anyone to code it for us or if I can figure out how to add it in, but for now, I'm afraid I'll have to ask you to assume that it's not going to happen, because while several of us have the ability to do simple stuff with PHP none of us are that skilled.
What I generally do though, is this: I pretend to start a new thread somewhere else in the forum, and then I just paste my ad content in there and 'preview' it until I'm happy with it. When I'm happy with it, I open a new window and go to the Classifieds and post the ad then. The only problem here is the possibility of accidentally clicking 'post' instead of 'preview' -- but so long as you haven't entered an actual title for the new thread, the forum won't let you post it, so you should be okay.
(And if you do accidentally post it somehow, you're welcome to edit the content to read "aww hell, I goofed up" and PM me requesting a delete.
) I realise it's a bit convoluted but hey, it works!
I'm really sorry that we can't offer this capability at the moment! I've wanted this for ages too.